Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing AlO2 and/or SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared, are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
As used herein, zeolites may be referred to by proper name, such as UZM-39, described in U.S. Pat. No. 8,992,885, or by structure type code, such as TUN. These three letter codes indicate atomic connectivity and hence pore size, shape and connectivity for the various known zeolites. The list of these codes may be found in the ATLAS OF ZEOLITE FRAMEWORK TYPES, which is maintained by the International Zeolite Association Structure Commission at http://www.iza-structure.org/databases/. Zeolites are distinguished from each other on the basis of their composition, crystal structure and adsorption properties. One method commonly used in the art to distinguish zeolites is x-ray diffraction. UZM-55 is a zeolite with a heretofore never before described structure.
Fecant and Bats describe in U.S. Pat. No. 8,361,435 the synthesis of a product they call IZM-2 from the crystallization of a gel comprising at least one organic species R containing two quaternary nitrogen atoms with a particular XRD pattern and having a SiO2/Al2O3 ratio preferably in the range from 60 to 600. The present invention involves a particular XRD pattern and has a SiO2/Al2O3 ratio of greater than 75, preferably greater than 100 and most preferably greater than 150.
The xylenes, para-xylene, meta-xylene and ortho-xylene, are important intermediates that find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid that is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates, and further comprise ethylbenzene, which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20% to 25% of a typical C8 aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non-equilibrium mixture of the xylene isomers that is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations.
In general, these xylene isomerization processes comprise contacting the xylene isomer sought to be isomerized with an isomerization catalyst under isomerization conditions. Various catalysts have been proposed for xylene isomerization. These catalysts include molecular sieves, especially molecular sieves contained in a refractory, inorganic oxide matrix. U.S. Pat. No. 4,899,012 discloses an alkylaromatic isomerization process based on a bimetallic pentasil-type zeolitic catalyst system that also produces benzene. U.S. Pat. No. 4,962,258 discloses a process for liquid phase xylene isomerization over gallium-containing, crystalline silicate molecular sieves as an improvement over aluminosilicate zeolites ZSM-5, ZSM-12 (MTW-type), and ZSM-21 as shown in U.S. Pat. No. 3,856,871. The '258 patent refers to borosilicate work, as exemplified in U.S. Pat. No. 4,268,420, and to zeolites of the large pore type such as faujasite or mordenite. U.S. Pat. No. 5,744,673 discloses an isomerization process using beta zeolite and exemplifies the use of gas-phase conditions with hydrogen. U.S. Pat. No. 5,898,090 discloses an isomerization process using crystalline silicoaluminophosphate molecular sieves. U.S. Pat. No. 6,465,705 discloses a mordenite catalyst for isomerization of aromatics that is modified by an IUPAC Group III element. U.S. Pat. No. 6,143,941, for instance, discloses oil dropped catalyst structures for xylene isomerization in which various molecular sieve structures are suggested including the MFI, MEL, EUO, FER, MFS, MTT, MTW, TON, MOR and FAU types of zeolites. The catalysts also contain a platinum group metal which may exist in the catalyst as the metal or as a compound such as an oxide, sulfide, halide or oxysulfide. U.S. Pat. Nos. 3,856,872; 4,899,011; 4,939,110 and 6,797,849 disclose, inter alia, MTW-type zeolites for xylene isomerization wherein the catalysts can contain at least one hydrogenation catalyst component.
Desirably the isomerization process performs as close to equilibrium as practical in order to maximize the para-xylene yield; however, associated with this is a greater cyclic C8 loss due to side reactions. The approach to equilibrium that is used is an optimized compromise between high C8 cyclic loss at high conversion (i.e., very close approach to equilibrium) and high utility costs due to the large recycle rate of unconverted C8 aromatics. Catalysts thus are evaluated on the basis of a favorable balance of activity, selectivity and stability.
Due to the large scale of commercial facilities to produce para-xylene on an economically competitive basis, not only must a xylene isomerization process be active and stable, but it also must not unduly crack the aromatic feed so as to result in ring loss. Moreover, the isomerization processes produce by-products such as benzene, toluene, and aromatics having 9 or more carbon atoms. For instance, U.S. Pat. No. 6,872,866 discloses a liquid phase process using two catalysts for the isomerization of xylenes and ethylbenzene. The catalysts comprise beta zeolite and low Si/Al2 MTW.
Often the xylene-containing feed to be isomerized also contains ethylbenzene. Ethylbenzene may be dealkylated such as would occur in the processes of U.S. Pat. No. 6,872,866, or the ethylbenzene can be converted. Advantageously, isomerization processes would convert ethylbenzene to xylenes. Whether the isomerization process will dealkylate or will convert ethylbenzene depends upon the isomerization process conditions including catalyst.
Catalysts for isomerization of C8 aromatics ordinarily are classified by the manner of processing ethylbenzene associated with the xylene isomers. Ethylbenzene is not easily isomerized to xylenes, but is normally converted in the isomerization unit because separation from the xylenes by superfractionation or adsorption is very expensive. A widely used approach is to dealkylate ethylbenzene to form principally benzene while isomerizing xylenes to a near-equilibrium mixture. An alternative approach is to react the ethylbenzene to form a xylene mixture via conversion to and reconversion from naphthenes in the presence of a solid acid catalyst with a hydrogenation-dehydrogenation function. The former approach commonly results in higher ethylbenzene conversion, thus lowering the quantity of recycle to the para-xylene recovery unit and concomitant processing costs, but the latter approach enhances xylene yield by forming xylenes from ethylbenzene. A catalyst composite and process which enhance conversion according to the latter approach, i.e., achieve ethylbenzene isomerization to xylenes with high conversion, would effect significant improvements in xylene-production economics.